The present invention relates generally to clutter reduction in synthetic aperture radar images. More particularly, the invention relates to the use of synthetic subapertures to extract moving target content from a single radar pulse stream.
The detection of moving targets is of critical interest in military applications, and may have some commercial applications such as traffic control. Important military targets, however, can have range rates that are quite small. For example, some targets have range rates on the order of five meters per second and less. Such low range rates can make it difficult to use airborne and spaceborne radar to detect the targets moving on the ground against most clutter backgrounds. Furthermore, sometimes the moving targets reflect weaker signals than the surrounding clutter and are even more difficult to detect without an effective clutter cancellation scheme.
Synthetic Aperture Radar (SAR) imagery is effective when the area to be imaged is located far away from the trajectory of the radar platform. In such a case, the target and clutter azimuth (defined as the angle from the radar velocity vector) will not be near either zero or 180 degrees. The zero and 180 degree ranges can be addressed with doppler radar techniques, whereas subaperture processing typically fills the gap when the azimuth nears 90 degrees. In some geometries, clutter is reduced using doppler processing alone. The reduction exploits the fact that the range-rate-doppler (or range-range-rate) profiles are different for the target content and the clutter content. When the radar beam azimuth is small, the ground clutter appears as a ridge in the range-rate-doppler plane. The target content can be located in the range-rate-doppler plane at a position that is separate from the clutter content ridge. There are conditions, however, in which doppler processing fails to remove the clutter content. As the target azimuth increases from zero, the clutter content ridge increases in width, making separation via doppler processing more difficult. Near ninety degrees azimuth, if the pulse repetition rate is equal to twice the maximum doppler spread in the beam, as required for simultaneous SAR, the clutter energy will fill the entire range-rate-doppler space. In such a case, separation via ordinary doppler processing is impossible.
Near ninety degrees azimuth, synthetic aperture radar (SAR) has been used to generate multiple SAR images and cancel the clutter content between the images. Generally, multiple radar pulse streams are received and each stream is used to generate an SAR image. Such an approach is often called the multiple-phase-center-antenna (MPCA) approach. The two-center version of this approach has been called DPCA. Another term for this concept is xe2x80x9cArrested SARxe2x80x9d, which implies SAR processing by itself. Existing versions of DPCA require physically separate phase centers. The essence of the MPCA concept is that two or more nearly identical representations of the ground clutter can be obtained by collecting radar pulse streams from different physical antenna phase centers. It is critical that the pulse rate and platform speed are arranged such that a successive phase center arrives at the exact same point in space that had been occupied by previous phase center. This spatial coincidence is achieved by designing the pulse rate to be equal to the platform speed divided by the phase center separation. The pulse data from each phase center can then be deramped and converted (by FFT, for example) into a plurality of SAR images. The result is that the pulse clutter content is identical from one phase center as compared with the corresponding pulse clutter content from the next phase center. By subtracting the pulse collected from one physical phase center from the pulse collected from the next phase center, the clutter is reduced. That is, one version of the clutter in one data set is subtracted from another nearly identical version in another data set. In the time it takes for the next physical phase center to arrive at the common spatial position, the target phase and amplitude may have changed enough for the target residual to be detectable.
Optimum clutter cancellation occurs when the phase of the return pulse signal from the target has changed by xcfx80 radians between phase centers, so that the target amplitudes will directly add upon pulse differencing. In this optimum case, the target actually increases by 6 dB while the clutter is reduced. On the other hand, no target residual survives if the target is not in motion relative to the clutter. In this case, it would be better to not attempt the clutter cancellation.
Under the above conventional approaches, spaceborne radar antennas are extremely expensive. Specifically, with the MPCA approach, the slower the target relative to the clutter the larger the displacement between the phase centers must be in order for the radar to detect the slowly moving target. Thus, the higher costs associated with the conventional approach are due to the need for very long physical antennas. As will be discussed below, the antenna size required to detect a target moving five meters per second or faster could exceed eighty meters using a conventional spaceborne SAR design. For space and airborne applications, an eighty meter antenna consisting of ten five-by-eight meter panels would cost approximately twenty to one hundred million dollars, depending upon whether future or current technologies are employed. The lower figure of twenty million dollars corresponds to future technology and is based on an estimate of approximately $15,000 per square foot of active antenna area. In contrast, a single five-by-eight meter panel will cost between four and twenty million dollars, again depending on the level of future technology employed. Further complexities, such as in the mechanical and electrical interface of multiple antenna panels increase costs further. Electrical compensation for the effects of channel equalization, signal delay and antenna mechanical deformation is likely to be more complex. The large antenna may require more processing power, data storage and down-link communications bandwidth.
The relationship between target range rate and required antenna length can be calculated as follows. Optimum cancellation occurs if the target range rate is equal to the radar signal wave length over the quantity:two times the pulse repetition interval. Consider a satellite in orbit with the parameters in the table below.
The width of the spot in azimuth on the ground at a 1e6 meter slant range, transmitted from an 8.3 meter aperture, is
sd=xcexr/d=3614 meters
The doppler bandwidth at that spot, and thus the required minimum pulse repetition frequency (PRF), is
bd=2VsSd/(rxcex)=1728 Hz
The 8.3 meter antenna can be divided into two phase centers with separation of 4.15 meters. This 4.15 meter separation corresponds to the satellite speed Vs divided by the PRF. This insures coincidence of phase centers for two consecutive pulses. The optimum target range rate for this configuration is
rdotopt=0.5xcexprf3600/1000=93.3 Km/hr
For this configuration, a target would be considered to be slow moving if the range rate were approximately one-tenth of the optimum target range rate. Thus, a range rate of five kilometers per hour would be slow moving for satellite tracking purposes. The target gain against the clutter would be reduced by roughly the sign of xcfx80/10, which will reduce the target to clutter gain by 10.2 dB. More phase centers can be added to increase the length of the antenna and optimally detect the five kilometers per hour target. It would take nineteen phase centers, however, or a total antenna length of 78.3 meters to optimally detect the slow moving target.
It is therefore desirable to provide ability to detect the presence of slow moving targets in a clutter background using subaperture processing. Furthermore, obviating the need for large antennas associated with MPCA approaches is highly desirable. Reducing size will in turn reduce the cost of antennas and thereby reduce the cost of satellites.
The above and other objectives are provided by a system and method in accordance with the present invention for extracting moving target content from a radar pulse stream. The method includes the steps of receiving the radar pulse stream from a single phase center antenna, and converting the radar pulse stream into a plurality of SAR images. Each image has a corresponding moving target content and a corresponding clutter content. The method further provides for canceling identical clutter content between the images. Converting a single radar. pulse stream into a plurality of SAR images allows slowly moving targets to be detected with substantially smaller and less expensive antennas.
Further in accordance with the present invention, a method for converting a radar pulse stream into a plurality of SAR images includes the step generating a plurality of synthetic subapertures based on the radar pulse stream. A common reference point is then defined, where the common reference point has known slant ranges to the plurality of synthetic subapertures. The method further provides for computing deramped signals for the synthetic subapertures based on the known slant ranges to the common reference point. SAR images can then be generated for the deramped signals.
The present invention also provides a radar tracking system having a single phase center antenna for receiving a radar pulse stream. The tracking system further includes a signal processing system for converting the radar pulse stream into a plurality of SAR images. Each image has a corresponding moving target content and a corresponding clutter content. The tracking system also includes a targeting system for canceling identical clutter content between the images.